The alkylation of an alkylation substrate (e.g., isoparaffin) with an alkylating agent (e.g., olefin) is an exothermic reaction. Commercially it is practiced using a catalyst at relatively low temperatures, which helps raise the yield of alkylate, the valuable product. The optimum alkylation temperature depends on many factors including the choice of catalyst, but alkylation reactors generally run in the range of −50 to 100° C. (−58 to 212° F.). The alkylation reactor effluent is usually in that temperature range too. Thus, the effluent poses a unique technical challenge in three aspects—to recover the product alkylate, to utilize the heat released by the alkylation reaction, and to recycle reactants to the reactor at a suitable low temperature.
Prior art alkylation units try to meet this challenge by flashing the reactor effluent. This produces a vapor that contains mostly unreacted alkylation substrate. This vapor is then used like a refrigerant is used in a refrigeration system; indeed, it is commonly called “refrigerant”. It is compressed, condensed, and then flashed. These steps provide a stream which is both coolant and reactant. It is, therefore, well-suited for recycling to the reactor.
Unfortunately, the prior art units don't work well when the vapor contains light components don't condense at conventional condensation conditions. These components often include (but are not limited to) hydrogen, hydrogen chloride, methane, and ethane. They are commonly called “noncondensables,” even though they most certainly will condense at very high pressure, if the temperature is low enough. But compressing the refrigerant in an alkylation process to that high of a pressure is prohibitively expensive. The costs of high-pressure equipment and utilities are simply too great.
Therefore, efficient methods are sought to recover alkylate, use the heat of reaction, and recycle unreacted alkylation substrate.